Exhaust Aftertreatment System with Pre-Catalysis

ABSTRACT

An exhaust aftertreatment system includes an exhaust tube or housing having a catalyzed inner wall catalytically reacting with engine exhaust prior to reaching an aftertreatment element.

BACKGROUND AND SUMMARY

The invention relates to aftertreatment systems for internal combustion engine exhaust, and more particularly to chemical species injection, and catalysis.

To address engine emission concerns, new standards continue to be proposed for substantial reduction of various emissions, including NO_(x) and particulate emissions. Increasingly stringent standards will require installation of aftertreatment devices in engine exhaust systems. Some of the aftertreatment technologies require certain chemical species to be injected into the exhaust system. For example, HC or fuel is injected in some active lean NO_(x) systems, and additives such as cerium and iron are injected for diesel particulate filter regeneration, and urea solution is injected in selective catalytic reduction (SCR) systems for NO_(x) reduction. These injected chemical species mix with exhaust gas before reaching downstream catalysts or filters.

BRIEF DESCRIPTION OF THE DRAWING

Sole FIGURE 1 is a side schematic view of an exhaust aftertreatment system in accordance with the invention.

DETAILED DESCRIPTION

FIGURE 1 shows an exhaust aftertreatment system 10 including an exhaust tube or housing 12 having an upstream inlet 14 for receiving engine exhaust from an internal combustion engine 16 as shown at arrow 18, and having a downstream outlet 20 for discharging the exhaust as shown at arrow 22. The exhaust tube carries exhaust to an aftertreatment element 24 in housing section 25 for treating the exhaust, for example an SCR (selective catalytic reduction) catalyst and/or a DOC (diesel oxidation catalyst) or other oxidation catalysts and/or a DPF (diesel particulate filter) or other particulate filter. An injector 26 injects chemical species into the exhaust tube and mixing with the exhaust prior to reaching aftertreatment element 24. For example, in one embodiment, aqueous urea solution is injected at injector or doser 26. The injected urea decomposes and hydrolyzes to ammonia to react with and reduce NO_(x) in the exhaust. For further description regarding exhaust aftertreatment systems, reference is made to the following U.S. Patents, incorporated herein by reference, namely U.S. Pat. Nos.: 6,449,947; 6,601,385; 6,604,604; 6,712,869; 6,722,123; 7,211,226. In the preferred embodiment, a mixer 28 is provided in the exhaust tube upstream of aftertreatment element 24 and mixing the exhaust and the injected chemical species, as is known, for example in the noted incorporated patents, for example a turbulator, impactor, flow deflector, flow diffuser, etc. It is desired that the injected chemical species be well mixed with exhaust gas before reaching aftertreatment element 24.

Exhaust tube 12 has a catalyzed inner wall 30, e.g. provided by a catalytic coating, catalytically reacting with the exhaust prior to reaching aftertreatment element 24. Such catalyzed inner wall is upstream of aftertreatment element 24 and downstream of injector 26. Aftertreatment element 24 is supported in housing section 25. It is preferred that inner wall 30 extend to and along housing section 25 at outward taper 27 to aftertreatment element 24. Exhaust flows axially along an axial flow direction 32 to aftertreatment element 24. Inner wall 30 extends axially and parallel to exhaust flow along axial direction 32 and then is tapered outwardly at taper 27 along housing section 25 to aftertreatment element 24. Aftertreatment element 24 extends across and spans housing section 25 transversely to axial direction 32. In one embodiment, mixer 28 is downstream of injector 26. In another embodiment, as shown in dashed line, mixer 28 a is upstream of injector 26. In a further embodiment, the mixer is catalyzed, e.g. with a catalytic coating, and catalytically reacts with the exhaust, for example as in commonly owned co-pending U.S. patent application Ser. No. 11/360,941, filed Feb. 23, 2006.

In the preferred embodiment, exhaust aftertreatment system 10 is an SCR, selective catalytic reduction, system for engine exhaust, including an exhaust tube or housing 12 having the noted upstream inlet 14 for receiving engine exhaust at 18, and the noted downstream outlet at 20 for discharging the exhaust at 22, an injector 26 for injecting urea reductant into the housing to hydrolyze to ammonia to react with and reduce NO_(x) in the exhaust, an SCR catalyst 24 in the housing catalytically reacting with the exhaust, and wherein housing 12 has a catalyzed inner wall 30 catalytically reacting with the exhaust prior to reaching SCR catalyst 24 and accelerating the rate of urea hydrolysis to ammonia and reducing urea crystalline formation on inner wall 30.

The system further enables and provides a desirable method for reducing housing length in an SCR system, namely by providing housing 12 with a catalyzed inner wall 30 catalytically reacting with engine exhaust prior to reaching SCR catalyst 24 and accelerating the rate of urea hydrolysis to ammonia and reducing the length of inner wall 30 otherwise required between injector 26 and SCR catalyst 24. The urea reductant is injected in the exhaust flow upstream of the SCR catalyst, and the urea decomposes and hydrolyzes to ammonia on or upstream of SCR catalyst 24. The decomposition and hydrolysis typically takes place over a sufficiently long exhaust tube upstream of SCR catalyst 24. The length of such exhaust tube upstream of the SCR catalyst to enable sufficient decomposition and hydrolysis may be objectionable in vehicles with limited space and packaging constraints. The present system provides a simple and effective way of reducing such length.

Current production SCR systems used on low temperature duty cycle applications may experience issues with crystallization of the urea, which builds up on the inner surface of the exhaust tube and can cause blockage or restriction of exhaust gas flow. The present system reduces the occurrence of urea crystalline formation on inner wall 30 due to the catalytic reaction thereat. The present system thus both: a) reduces the length required of the decomposition or hydrolysis exhaust tube; and b) reduces the occurrence of crystalline formation on the inner tube wall.

In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. The different configurations, systems, and method steps described herein may be used alone or in combination with other configurations, systems and method steps. It is to be expected that various equivalents, alternatives and modifications are possible within the scope of the appended claims. 

1-11. (canceled)
 12. An SCR, selective catalytic reduction, system for engine exhaust, comprising: a housing having an upstream inlet for receiving said exhaust, and a downstream outlet for discharging said exhaust; an injector injecting urea into said housing to hydrolyze to ammonia to react with and reduce NO_(x) in said exhaust; an SCR catalyst in said housing catalytically treating said exhaust; said housing having a catalyzed inner wall catalytically reacting with said exhaust prior to reaching said SCR catalyst and accelerating the rate of urea hydrolysis to ammonia and reducing urea crystalline formation on said inner wall.
 13. A method for reducing housing length in an SCR, selective catalytic reduction, system for engine exhaust having a housing having an upstream inlet for receiving said exhaust, and a downstream outlet for discharging said exhaust, an injector injecting urea into said housing to hydrolyze to ammonia to react with and reduce NO_(x) in said exhaust, and an SCR catalyst in said housing catalytically treating said exhaust, said method comprising providing said housing with a catalyzed inner wall catalytically reacting with said exhaust prior to reaching said SCR catalyst and accelerating the rate of urea hydrolysis to ammonia and reducing the length of said inner wall otherwise required between said injector and said SCR catalyst. 